1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus which is constructed to correct a change in the curvature of field of a lens caused by an operating temperature change so as to form a small-diameter optical beam spot on a surface to be scanned regardless of changes in the operating temperature.
2. Description of Related Art
An optical scanning apparatus is often provided in laser printers, digital copying machines or facsimile machines. It is also well known to use plastic material for manufacturing a lens to reduce the cost of the lens or to form a surface of the lens so as to have a specific shape. For a scanning image forming lens used for forming a deflected light flux into an image on a scanning surface, there are proposed various lens shapes for correcting the curvature of field and the constant velocity characteristics, and plastic material is known to be suitable for forming such lens shapes to achieve such correction of curvature of field.
The curvature of field and the refractive index of the surface of a plastic lens both change in accordance with a change in the volume of the lens due to an operating temperature change. As a result, the lens performance and especially the curvature of field changes in accordance with changes in the operating temperature. A change of the curvature of field causes an increase in the diameter of an optical beam spot formed on a scanning surface, and consequently, lowers the resolution of optical scanning and images formed by such scanning.
A change of the curvature of field of a plastic lens caused by a temperature change occurs in an opposite manner in positive and negative lenses. Accordingly, a change of the curvature of field of a plastic scanning image forming lens caused by a temperature change can be corrected by arranging another plastic lens having a power opposite to that of the scanning image forming lens along an optical path between a light source and an optical deflector. Such an arrangement offsets the change in the curvature of field of the plastic scanning image forming lens by having a suitable change in the curvature of field of another plastic lens, as disclosed in Japanese Patent Laid-open Publication Nos. 8-160330 and 8-292388.
In the optical scanning apparatus described in JP 8-160330 and 8-292388, the plastic lens arranged between the light source and the optical deflector does not have a power relative to a direction corresponding to a main scanning direction. Therefore, the plastic lens arranged between the light source and the optical deflector does not correct a change of the curvature of field in the direction corresponding to the main scanning direction which is caused by the changes in operation temperature. As a result, an increase in the diameter of the optical beam spot in the main scanning direction cannot be prevented. The direction corresponding to a main scanning direction herein refers to the direction corresponding a scanning direction of an optical scanning apparatus along an optical path from a light source to a scanning surface and the direction corresponding to a sub scanning direction refers to a direction corresponding to a sub scanning direction of the optical scanning apparatus along the same light path, which is substantially perpendicular to the main scanning direction.
Further, for preventing an increase of an optical beam spot diameter in the sub scanning direction, it is known that wave-front aberration must be considered not only for paraxial light fluxes but also for the entire light fluxes.
Preferred embodiments of the present invention overcome the problems described above by providing an optical scanning apparatus, which is constructed and arranged to include a plastic lens for a scanning image forming lens for forming a deflected light flux into an image on a scanning surface such that the plastic lens corrects any change in the curvature of field in both the main scanning direction and the sub scanning direction, which change in curvature of field is caused by a change in operating temperature, and to consequently form an optical beam spot having a very small diameter on the scanning surface regardless of changes in the operating temperature.
The preferred embodiments of the present invention also provide an optical scanning apparatus which is constructed to correct an effect of the change in the curvature of field relative to the paraxial light fluxes in the sub scanning direction and also to correct the wave-front aberration relative to the entire light fluxes in the sub scanning direction.
According to a specific preferred embodiment of the present invention, an optical scanning apparatus includes a light source, a first optical lens system, a second optical lens system, an optical deflector and a third optical lens system. A semiconductor laser or a light emitting diode (LED) may be used as the light source. The first optical lens system couples a divergent light flux emitted from the light source to a subsequent optical lens system, which is the second optical lens system. The first optical lens system converts the light flux emitted from the light source either to a parallel light flux or to a weak converging or weak diverging light flux. The second optical lens system forms the light flux emerged from the first optical lens system into a line image extending in a direction corresponding to the main scanning direction. The optical deflector has a deflecting reflective surface located near where the line image is formed and deflects the light flux reflected by the deflecting reflective surface. A rotating polygonal mirror, a rotating single-surface mirror or a rotating double-surface mirror may be used for the optical deflector. The third optical lens system condenses the deflected light flux on a surface to be scanned as an optical beam spot so as to scan the surface to be scanned. The third optical lens system preferably includes at least one plastic lens. The third optical lens system may include two or more lenses, or may be constructed by a combination of one or more lenses and a concave-shaped mirror having an image forming function. A flat-surface mirror may be arranged to bend a light path for the deflected light flux, in addition to the third optical lens system, between the optical deflector and the surface to be scanned, depending upon the arrangement of the optical scanning apparatus. The second optical lens system preferably includes at least one plastic lens and one glass lens.
According to one feature of preferred embodiments of the present invention, the surface of the plastic lens in the second optical lens system is formed such that a change in the curvature of field in the main scanning direction and in the sub scanning direction caused by a change in the volume of the plastic lens in the third optical lens system, which is caused by a change in the operating temperature, is accurately and completely corrected. Accordingly, the change in the curvature of field in the main scanning direction and the sub scanning direction, which is caused by changes in the operating temperature, is substantially prevented and thereby the diameter of the optical beam spot is kept very small.
In the above described optical scanning apparatus, the plastic lens of the third optical lens system may be configured to have a positive power in both directions corresponding to the main scanning direction and the sub scanning direction, respectively, and the plastic lens of the second optical lens system may be configured so as to be an anamorphic lens having a negative power in both directions corresponding to the main scanning direction and the sub scanning direction and the glass lens of the second optical lens system may be configured to have a positive power at least in the sub scanning direction. By constructing the second optical lens system as described above to achieve a synergistic effect of the glass and plastic lenses, the characteristics required for the second optical lens system can be distributed between the glass and plastic lenses. More specifically, the plastic lens of the second optical lens system is arranged and constructed to eliminate any effects of changes in the curvature of field in the third optical lens system caused by changes in an operating temperature. As a result, the design of the second optical lens system can be greatly facilitated and the excellent performance of the second optical lens system can be achieved relatively easily.
Further, the plastic lens of the second optical lens system may include at least one surface which has a non-arc shape in the sub scanning cross section. As a result, changes in the curvature of field in the main scanning direction and the sub scanning direction caused by changes in the operating temperature are substantially prevented and further, the spherical aberration in the sub scanning direction is reduced. Accordingly, the diameter of the optical beam spot is kept very small and has a much more uniform and constant size. It is also possible for the plastic lens of the second optical lens system to include at least one surface which has a non-arc shape in both the main scanning direction and the sub scanning direction.
Furthermore, the plastic lens of the second optical lens system may include a surface which has a symmetrical shape relative to the optical axis and which has a negative power. Thereby, treatment and measurement of the plastic lens are facilitated and the eccentric tolerance of the plastic lens is increased.
According to another specific preferred embodiment of the present invention, the third optical lens system functioning as a scanning image forming lens includes at least one plastic lens which has a positive power at least in the direction corresponding to the sub scanning direction. The second optical lens system includes at least one plastic lens and one glass lens as in the above described preferred embodiment. The plastic lens preferably has a negative power at least in the direction corresponding to the sub scanning direction and has at least one surface having a non-arc shape in the sub scanning cross section. The glass lens preferably has a positive power at least in the sub scanning direction. Therefore, any change in the curvature of field in the sub scanning direction caused by changes in the operation temperature is substantially prevented and the spherical aberration in the sub scanning direction is greatly decreased. Accordingly, the diameter of the optical beam spot is kept very small.
Further, in any of the above described preferred embodiments, the third optical lens system may be constructed by one plastic anamorphic lens having positive powers which are different in the main scanning direction and the sub scanning direction and the second optical lens system may be constructed to include one plastic and one glass lenses. Thus, the total number of lenses in the optical scanning apparatus may be reduced and consequently, the construction and the assembly of the optical scanning apparatus are simplified and the cost of the apparatus is significantly decreased. Moreover, because the plastic lens can be made by a mold and can be easily formed to have a desired shape, it is relatively easy to form the plastic lens so as to have a desired non-arc shape.
According to still another specific preferred embodiment of the present invention, the third optical lens system includes at least one plastic lens having a positive power at least in the direction corresponding to the sub scanning direction. The second optical lens system is configured to include at least two lenses, one made of plastic and the other made of glass. The plastic lens of the second optical lens system preferably has a negative power in the direction corresponding to the sub scanning direction, and in addition, has at least one surface having a non-arc shape in the main scanning cross section and one surface having a non-arc shape in the sub scanning cross section. The glass lens has a positive power at least in the direction corresponding to the sub scanning direction. The glass lens may have a power in the direction corresponding to the main scanning direction also. Because of the above configuration including the surface having a non-arc shape in the main scanning direction, the plastic lens of the second optical lens system also has a power in the main scanning direction.
The above power of the plastic lens of the second optical system in the direction corresponding to the main scanning direction may be set so as to be opposite to that of the plastic lens of the third optical lens system. That is, the plastic lens of the second optical lens system may include a negative power in the direction corresponding to the main scanning direction also. When the plastic lens of the second optical lens system includes a negative power in the direction corresponding to the main scanning direction, the plastic lens of the third optical lens system (scanning image forming lens) is positive.
In the preferred embodiment described in the previous paragraph, the plastic lens of the second optical lens system may be configured so as to include a surface having a negative power only in the direction corresponding to the sub scanning direction and a non-arc shape in the sub scanning cross section and a surface having a negative power only in the direction corresponding to the main scanning direction and a non-arc shape in the main scanning cross section. Further, the plastic lens of the second optical lens system may be configured to have a surface having a negative power only in the direction corresponding to the sub scanning direction and a non-arc shape in the sub scanning cross section, and a coaxial non-spherical surface, which is formed to have a symmetrical shape relative to the optical axis.
These and other features, advantages and elements of the present invention will be apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings as described below.